The present invention relates to thermal spraying, and particularly to improved guns for spraying metallic and ceramic particles onto a substrate. More particularly, the present invention relates to water-cooled thermal spray guns having an anode.
Plasma-arc spray guns use a power supply and a cathode disposed within an anode to generate a plasma for use in depositing a material onto a substrate. A gas supplied to the chamber between the anode and the cathode converts to high-temperature plasma as it passes through an arc that extends between the anode and cathode. To provide for stable and controllable plasma, it is important to control the location of the arc between the anode and cathode. To that end, other anodes contain a series of cylindrical and frustoconical sections designed to position the arc at the desired point. However, these contours produce undesirable turbulence behind the arc attachment point and reduce the performance of the gun.
The large currents of electricity flowing between the anode and the cathode cause the anode to heat significantly, thereby reducing its performance and operating life. To control the heating and reduce anode damage, a cooling-water flow passes around and within the anode. Present plasma-arc spray guns employ water channels that have multiple chambers and flow paths with differing flow areas. Rapid increases in flow area cause sudden pressure drops that can be detrimental to the cooling efficiency of the water flow. More specifically, the pressure drop allows the water to boil and greatly reduces its cooling effectiveness.
Another factor in the determination of anode life is the wall thickness of the anode. Large changes in wall thickness in adjacent sections can result in significant thermal stress and component failure. In addition, varying wall thickness can result in significantly different heat transfer characteristics causing hot spots or cold spots on the surface of the anode.
Thus, the plasma-arc spray gun of the present invention provides a cathode and an anode defining a longitudinal axis. The anode further includes an external surface and an internal chamber, the internal chamber extending from a first end to a second end. At least a portion of the internal chamber is defined by revolving a non-linear curve about the longitudinal axis. The plasma-arc spray gun also includes a gun body supporting the cathode and the anode.
In another construction of the plasma-arc spray gun the gun is powered by an external power source having a first lead and a second lead. The gun provides a gun body and an anode supported by the gun body and electrically connected to the first lead of the power source. The anode also has a longitudinal axis and includes an external surface and an internal chamber. The internal chamber has a first open end receiving a flow of gas and a second open end discharging a flow of plasma. The internal chamber also includes a portion defined by revolving a non-linear curve about the longitudinal axis. The plasma-arc spray gun further includes a cathode supported by the gun body and electrically connected to the second lead of the power source and a gas injector providing the flow of gas through the first open end of the anode. The power source initiates an arc between the anode and the cathode, and a portion of the flow of gas passes through the arc to generate the flow of plasma.
In preferred embodiments, the non-linear curve is defined by a polynomial equation. In addition, the non-linear curve is disposed between the first open end of the anode adjacent the gas injector and the arc attachment area.
The invention further provides a method of manufacturing a plasma-arc spray gun. The method comprises the steps of forming an inner chamber within an anode having a longitudinal axis. The inner chamber includes a first open end, a second open end, and at least one region disposed therebetween and defined by the revolution of a non-linear curve about the longitudinal axis. The method further includes the steps of positioning the anode and the gas injector within the gun body and positioning the cathode at least partially within the inner chamber of the anode.
In other embodiments, the method further comprises the step of forming an external anode surface defined by the revolution of a second non-linear curve about the longitudinal axis. The second non-linear curve is substantially parallel to and spaced apart from the first non-linear curve.